1. Field of the Invention
The invention relates to a light amount adjusting apparatus which is used in an image pickup apparatus such as video camera, digital still camera, or the like and to an optical apparatus having such a light amount adjusting apparatus.
2. Related Background Art
A light amount adjusting apparatus for adjusting a light amount upon photographing is used in an image pickup apparatus such as a video camera or the like. FIG. 10 shows a construction of such a conventional light amount adjusting apparatus. According to such an apparatus, two aperture blades are linearly driven in the opposite directions, thereby enabling an aperture opening diameter to be changed.
In FIG. 10, reference numeral 400 denotes an aperture driving source (motor). An output axis 401 is rotated by supplying a current to a coil of the motor 400. An interlocking lever 402 is integratedly fixed to the output axis 401. The lever 402 has blade interlocking pins 403 and 404 at positions which are away from a rotary axis at a same distance R, respectively.
Reference numerals 405 and 406 denote aperture blades. An aperture opening 413 is formed by those two aperture blades. An ND filter 414 is integratedly adhered and fixed to the aperture blade 406. A part of the aperture opening 413 is covered with the ND filter 414.
Reference numeral 415 denotes an optical axis. Hole portions 407 and 408 adapted to be fitted to the foregoing blade interlocking pins 403 and 404 are formed in the aperture blades 405 and 406, respectively. Thus, the interlocking lever 402 and aperture blades 405 and 406 are interlocked.
Long hole portions 409 and 412 of the aperture blades 405 and 406 are guided by guide pins formed on an aperture base (not shown), so that the aperture blades 405 and 406 are moved in the guiding direction in association with the rotation of the interlocking lever 402, thereby enabling the aperture opening diameter to be varied.
For example, when the interlocking lever 402 is rotated in the direction shown by an arrow 416 in the diagram, the blade interlocking pin 404 moves downward and the blade interlocking pin 403 moves upward. In association with it, the aperture blade 406 moves downward and the aperture blade 405 moves upward, so that the aperture opening diameter (that is, an area of aperture opening) changes in such a direction as to increase more. So long as the rotating direction of the interlocking lever 402 is opposite to the direction of the arrow 416, the aperture opening diameter changes in such a direction as to decrease more.
When the aperture opening diameter decreases, a light diffraction phenomenon occurs easily.
Therefore, in order to remove an adverse influence by such a diffraction on the small aperture side, in the light amount adjusting apparatus of FIG. 10, the ND filter 414 is attached to the aperture blade 406. The occurrence of the diffraction phenomenon is prevented by reducing light transmittance as well as the reduction of the aperture opening diameter.
However, in the light amount adjusting apparatus of the construction such that the ND filter is attached to the aperture blade, in the case where an F value at which the small aperture diffraction occurs becomes brighter in a recent image pickup device such as a CCD or the like having very small pixels, it is necessary to further decrease the transmittance of the ND filter. In case of using the ND filter whose transmittance is further decreased, there is a new problem such that an illuminance variation occurs in a picture plane in a state of the opening diameter where the ND filter covers almost the half of the aperture opening.
As mentioned above, hitherto, although the ND filter has been provided for the aperture blade in order to eliminate an image deterioration due to the light diffraction that is caused in case of the small aperture, it is impossible to avoid the illuminance variation and the resolution deterioration in association with the miniaturization of the image pickup device such as a CCD or the like and the realization of a high density of the pixels thereof.